Special effects such as highly reflective metallic-effect packaging is often used in retail and promotional packaging designs to promote, attract and draw increased customer attention and thereby increase sales and services of packaged products. Toward this end, traditional printing methods may utilize a pre-processed metallic foil or metalized film laminated to paper or paperboard substrate such as used to produce folding carton boxes for goods. In this method, the metallic substrate is relatively expensive compared to standard quality white paper or paperboard used for printing standard quality graphic designs for folding carton packaging. In addition, metallic foil or metalized film laminated paper or paperboard is typically overprinted over a significant surface area with opaque white ink covering the metallic effect, to provide sufficient area of a neutral white base-coat for colored graphics to be printed, while leaving less than 100% of the metallic surface uncovered to create the intended visible metallic effect.
In a more efficient approach, packaging requiring a metallic effect can be printed only in the required regions on standard quality white coated paper or paperboard using a metallic effect printing ink. Low viscosity liquid printing inks applied onto porous and non-smooth substrates penetrate and soak into the porous substrate, and/or dry in a non-smooth layer following the non-smooth surface contours of the substrate, resulting in a visible and measurably low-reflectivity, non-aesthetically pleasing metallic appearance compared to the high reflective qualities of metallic foil or metalized film. The use of primers is typically recommended in order to help improve the metallic appearance.
Although the concept of using a primer to improve the surface of a substrate before applying an ink is not new, there is very limited prior art related to energy curable primers that are effective at improving metallic appearance. A recent patent application from Shorewood Packaging/International Paper (WO2012099698) discloses a method to produce a package with a metallic appearance including applying an energy curable primer and a metallic ink. However, there is no information provided regarding the composition of suitable primers that will enable the creation of packages having the desired metallic appearance to replace foil stamping or use of fully metalized board such as board laminated with a metalized film (“Met-Pol” board) or board laminated with a thin aluminum foil (“Transmet” board).
Achieving superior reflectivity and brilliance index requires special primer characteristics which are not disclosed by Shorewood. In addition, metallization primers disclosed in the prior art, such as the clear undercoat primer from Jetrion disclosed in US2007/076069 and U.S. Pat. No. 7,891,799, do not provide the same level of temperature dependency of the viscosity exhibited by the primers of the present invention. This particular rheological behavior in the primers of the present invention brings superior leveling capability at application temperature with reduced board penetration. As a result, the primer has superior gloss retention over a large range of anilox volume (which relates to applied coating weight) going from 15 bcm (billion cubic microns/in2) to 10 bcm and 5 bcm. The primers of the present invention preferably have a viscosity reduction % of over 55% between 25° C. and 45° C. whereas the prior art primer typically shows about 27%.
In addition, primers from the prior art are very sensitive to the surface they are printed onto as illustrated in the gloss differences on BYK Penopac charts between the primer on the white coated part of the chart compared with the white uncoated part of the chart. That sensitivity is further increased with lower anilox volumes as seen illustrated in Example 2 below.
Accordingly, the prior art also fails to disclose primer formulations which would be suitable for use in packaging where organoleptic characteristics such as odor and migration are important (e.g. food, tobacco, and pharmaceutical packaging). By selecting acrylate monomers and oligomers having certain properties and/or with a MW of preferably at least 300 g/mol in combination with appropriate additives and, if applicable, photoinitiator package, one can produce an effective metallization primer with low odor and low set-off migration characteristics (per testing from EU directive No. 10/2011).